A wide range of different electronic projectors employ light modulating panels of different kinds to generate the image which by using lens optics is projected onto a screen. Currently the mostly used panels are of types LCD (“Liquid Crystal Display”) or DMD (“Digital Modulating Device”).
An LCD-panel is made of a number of light valves arranged side by side in a matrix, wherein each light valve is individually controllable and generates a point or image element, frequently referred to as a pixel, in an image assembled by the pixel light valves of the matrix. By controlling the light valve of each pixel to let through more or less light from a rewards located light source, seen from the direction in which the light flows from the light valve of the pixels, an image will be formed assembled by the pixels which let through light in mainly the same direction.
A DMD-panel is mainly generated by a number of tiltable micro mirrors, arranged side by side in a matrix, wherein each micro mirror is individually controllable and generates a point or a picture element, frequently referred to as a pixel, in an image consisting of a large number of such pixels. By controlling individually the mirrors of the various pixels to reflect light from a forward located light source in a particular direction, seen from the direction into which light is reflected, an image is generated assembled by the points that reflect light mainly in the same direction. The optical efficiency of a DMD-panel is frequently improved by combining the DMD-panel with a TIR-prism (total internal reflection prism).
By the commonly employed light source which emits light with a spatial distribution, such as for example an incandescent lamp or an arc lamp, which may include a kollimator, the light flowing from each pixel exhibits a divergence corresponding to the divergence of the light provided by the light source to the panel.
To project onto a screen the image generated by the light modulating panel, a lens is arranged in the path of the light exiting from the panel, such that light is collected, i.e. focused, in a plane of projection, which represents an imaging of the panel itself onto the screen.
Due to limitations in the production technology, light modulating panels are delivered in s different and limited matrix sizes and dimensions, preferably adapted to standardized picture formats that often are used in other image acquisition technology or screen technology. The available matrix sizes and dimension thus control the image resolution that by one single light modulating panel is achievable in a projected image. As an example, with a panel which includes 800 pixels horizontally and 600 pixels vertically, a projected image to fill a cinema screen having dimensions 4000 mm horizontally and 3000 mm vertically, would exhibit pixels that each measures 5 mm by 5 mm. For a viewer located close to the screen, such as for example a viewer located in one of the first rows in the movie theatre, each pixel will be clearly visible and would destroy the viewing experience of the picture being projected.
To obtain a better resolution, a plurality of panels are employed in an arrangement for their projection side by side in the projection plane as mosaic elements that together form the desired, projected image. With the movie theatre screen example mentioned above, an image mosaic obtained using a two by two panel arrangement of the same panel type indicated above, would require only one half of the image magnification in the projection optics, and would provide an image which exhibits pixels each having the dimensions 2.5 mm by 2.5 mm. In other words, thereby a reduction by a factor of two of the pixel size in the projected image is achieved, which may prove to be acceptable for use in a movie theatre.
In the technical field of electronic image projection a number of different solutions are know for generating a projected image that includes a mosaic of imagings of a plurality of light modulating panels. Known solutions employ partly reflecting mirrors or polarization dependent optical elements to deliver light from light sources or for combining light exiting from a number of light modulating devices to generate an image mosaic in the image projection plane.
The technical field of the invention particularly relates to electronic image projectors of a type that make use of a single lens for imaging a number of light modulating panels for creating the image mosaic, and which preferably also includes a single light source for providing light to the light modulating panels of the projector.
A solution for a projector of the type referred to in the section above, is illustrated using the example of the accompanying FIG. 1. A light source OS1 transmits a ray bundle in a direction towards the partly transmittive mirror DMS and the mirror MR1, which in turn each direct their part of the ray bundle towards each of their light modulating devices OM1 and OM2, respectively. The pixels of the light modulating devices OM1 and OM2 forward the light exiting from the devices in a direction by the deflection of mirrors MR3 and MR2, respectively, which control the outwards flowing light towards and through the imaging lens LE1 and forwards towards the image plane IP where images IM1 and IM2, respectively, are generated side by side, by the respective panels of the light modulating devices OM1 and OM2. To generate colour images, a rotating colour filter CF1 is used, which is synchronised with the controlling of the light modulating panels of the light modulating devices OM1 and OM2.
FIG. 1 also illustrates that the lights which flow from the light modulating devices is OM1 and OM2, respectively, to generate an image mosaic comprised of the respective image parts IM1 and IM2 that are located immediately adjacent to each other, would only partly fall onto the mirrors MR3 and MR2, respectively, where the mirrors come together with the optical central axes of the lens. Such an arrangement would require strict control of the design of the mirror combinations MR3, MR2, to avoid a visible transition in the imagings of IM1, IM2 in the image mosaic. Furthermore, due to the spreading of the light coming from the light modulating devices OM1 and OM2, a considerable part of the light that is to generate the respective parts of the images IM1 and IM2 that are located close to the transition, would pass below the mirrors MR3, MR2, and lead to a significant vignetting.
A corresponding problem is also present in a projector that is designed according to the exemplary alternative that is illustrated in FIG. 2.
A possible solution to the above mentioned problems of the transition between the mirrors MR3 and MR2, and also to the vignetting which in the accompanying FIGS. 1 and 2 in part is indicated by the broken line, is to image the mosaic elements IM1 and IM2 with some overlap, and to control in a special way those parts of the panels of the light modulating devices OM1 and OM2 that contribute to generating the images in the overlapping area. Such a solution is, however, complicated as the control of the panels of the light modulating devices OM1 and OM2 would have to be adapted individually and be maintained adjusted and controlled for each and every projector optics.
A further possible solution, which may be understood by studying the solution suggested in FIG. 3, is to replace the mirror MR3 that is included in FIGS. 1 and 2, with an ordinary partly reflective mirror DM2 without any selectivity with regard to the wavelength of the light. A disadvantage of this solution wherein DM2 is an ordinary partly reflective mirror or beam splitter without any wave length selectivity, is that DM2 introduces a loss of light, as light exiting from OM1 that in part will be coupled through DM2, cannot be transmitted towards the lens LE1, and light exiting from OM2 that only in part will be reflected by DM2 also will not be forwarded towards the lens LE1. The light loss from OM1 is illustrated by the broken line U1 and the light loss from OM2 is illustrated by the broken line R2.